Technical Field
The present invention relates to a switching power supply with a feature for switching the output voltage according to the state of a load.
Background Art
Flyback-type switching power supplies have attracted attention as power supplies for driving low capacity electrical power loads. As illustrated in FIG. 3, this type of switching power supply typically includes a switching element Q that turns ON and OFF the current flowing through a primary coil P of a transformer T to which an input voltage Vin is applied. This switching element Q is a power MOSFET, an IGBT device, or the like. These types of switching power supplies are also equipped with a rectifying and smoothing circuit that rectifies and smooths the voltage induced in a secondary coil S of the transformer T to generate a DC output voltage Vout. The switching element Q is turned ON and OFF according to this output voltage Vout using feedback control.
A control circuit IC that controls the switching element Q typically turns the switching element Q off when the current flowing while the switching element Q is on produces a certain feedback voltage Vfb calculated according to the output voltage Vout. Then, when the current flowing through the transformer T inverts while the switching element Q is off, the control circuit IC turns the switching element Q back on to achieve quasi-resonance in the current flowing in the transformer T.
The feedback voltage Vfb is detected as the difference between a prescribed reference voltage Vref and a detected voltage Vsens detected by resistively dividing the output voltage Vout and is then fed back into the control circuit IC using a photocoupler PC, for example. Moreover, a supply voltage VCC for the control circuit IC is generated using a supply voltage generation circuit REG that includes a diode D and a capacitor C that rectify and smooth a voltage induced in an auxiliary coil A, for example.
In this type of switching power supply, the output voltage Vout is changed according to the state of the load in order to reduce power consumption. More specifically, an output voltage switching control scheme is implemented such that if the output voltage Vout during normal loading is 32V, for example, the output voltage Vout is switched to 12V during light loading or when no load is present, or if the output voltage Vout during normal loading is 24V, the output voltage Vout is switched to 8V during light loading or when no load is present, for example. This output voltage switching control scheme reduces the standby power consumption of the switching power supply. As illustrated in FIG. 4, changes in power consumption Pin of the switching power supply due to changes in the output voltage Vout follow a trend in which as the output voltage Vout decreases, the power consumption Pin also decreases.
In conventional technologies such as the example illustrated in FIG. 3, this type of output voltage switching control scheme was implemented entirely by changing the voltage-dividing ratio of resistors Ra and Rb that obtain the detected voltage Vsens by dividing the output voltage Vout according to external control signals. More specifically, an output voltage switching circuit VOSW receives the external control signals and turns ON and OFF a switching element (a transistor) Tr to selectively connect a resistor Rc in parallel to the resistor Ra, thereby changing the detection ratio between the detected voltage Vsens and the output voltage Vout. Therefore, the output voltage Vout can be increased by turning the switching element Tr on and decreased by turning the switching element Tr off. Patent Document 1, for example, discloses an example of this type of output voltage switching control scheme in detail.